I guess it depends on your business model, but I prefer to go with an approach similar to what Kevin has done. My business manages the entire process from copywriting and design to coding, deployment and conversion optimization, both for desktop and mobile landing pages.
If you want a scalable business I'd recommend moving away from charging by the hour. There are only 24 hours a day, so that automatically limits your potential for revenue. You will be better off selling a 'product' that gives you infinite scalability.
I charge by month with an additional small startup-fee for the initial setup. That it's easy for the customer to know how much they pay for the monthly optimization while I make sure to get paid for the initial work.
Hope this helps. Feel free to call if you need more tips and advice.